Nitrogen fixation can be achieved either biologically or through a nonbiological process. Industrial nitrogen fixation currently involves the catalytic combination of molecular nitrogen and molecular hydrogen into ammmonia at high pressure (.about.350 atm) and high temperature (.about.500.degree. C.) via the well-known Haber process. Ammonia is then used directly as a fertilizer, or it is converted to other useful reduced or oxidized nitrogen compounds.
Biological nitrogen fixation occurs by the action of certain very limited classes of bacteria; these bacteria are sometimes associated with the root systems of certain plants such as soybean, alfalfa, clover, tropical herbs, and aquatic ferns.
E. E. Van Tamelen and co-workers have reported (Journal of the American Chemical Society, Vol. 90, page 4492 (1968) and Vol. 91, page 5194 (1969)) the electrolytic reduction of N.sub.2 to NH.sub.3 using a non-aqueous electrolyte of glyme (1,2-dimethyoxyethane), aluminum chloride, and titanium tetraisopropoxide. An external voltage of 90 volts was applied across two platinum electrodes while N.sub.2 was bubbled through the electrolyte. Upon hydrolysis of the electrolyte, ammonia was recovered in a 10% yield based on the total titanium originally present.
A report on a process for photoelectrolytic molecular nitrogen fixation employing TiO.sub.2 powder is found in Chemical and Engineering News, Oct. 3, 1977, page 19.
G. N. Schrauzer et al. in Journal of the American Chemical Society, Vol. 99, page 7189 (1977) disclose photolysis of water and photoreduction of nitrogen on titanium dioxide powder.
In U.S. Pat. No. 4,011,149 the photoelectrolytic dissociation of water into hydrogen and oxygen is disclosed. This process, called photoelectrolysis, involves the conversion of optical energy into chemical energy through an endoergic chemical reaction using photoactive semiconductor electrodes.
Steven N. Frank and Allen J. Bard in Journal of the American Chemical Society Vol. 99, page 4667 (1977) broadly suggest that photoassisted reductions at p-type materials can be carried out to produce new materials with light, rather than electrical or chemical energy, supplying the driving force for the reaction.
In accordance with private information received in a letter dated May 19, 1978 from M. Halmann, Associate Professor, The Weizmann Institute of Science, Rehovot, Israel, the writer of the letter has been working on an unspecified novel application of p-type semiconductor electrodes for photoassisted reduction reactions in the field of organic chemistry.
A photoelectrolytical process is characterized by the conversion of optical energy into chemical energy. A photocatalytic process is characterized by the conversion of optical energy into the activation energy required to drive the chemical reaction.
The prior art processes for nitrogen fixation under mild, ambient conditions are relatively inefficient. Furthermore, the prior art electrolysis processes require either an external voltage source or a strong reducing agent, such as sodium or potassium metal, which is expended during the process.